I need to flip the screen of Mac X 10.4 either vertically or horizontally. I should emphasis that I do not want to rotate 180 degree I want to have mirrored output. Is there any way of driving a Mac's display as a mirror image (and not simply rotated)?

Several answers have jumped on rotation, so please don't confuse rotation with actual flipping. A mirror image (either vertical or horizontal) distorts text in a similar manner to taking a sheet of nearly transparent paper and literally flipping it so you see the words from behind the paper.

Here is an illustration of rotation (not what this question seeks)
: rotate display 90, 180 or 270 degrees.

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Do you need to flip an built-in display (like on an iMac or MacBook), or an external display?

I'm afraid it's only possible to flip an external display. Here's how to do that (tested with 10.6, but 10.4 should be similar):
Go to System Preferences-> Displays. Go to the Display tab of your external monitor's window. For Rotation: select either 90 or 270.